A Theory-guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition Metal Catalysts

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically-dispersed Pt supported on crystalline MgO (chosen for its well-defined bonding sites for Pt) as a prototypical example, in this work, we demonstrate how systematic density functional theory calculations (for assessing all the potentially stable Pt sites) combined with automated EXAFS analysis can lead to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging, EXAFS, and HERFD-XANES spectroscopies; the proposed Pt site are in full agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active sites than what is currently possible with conventional EXAFS analysis. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Local electronic and atomic structures of the mixed B-site ions in SrFe1−xMnxO3−δ studied with X-ray absorption spectroscopy

Local electronic and atomic structures of the B-site ions in perovskite SrFe[Formula: see text]Mn[Formula: see text]O[Formula: see text] were evaluated with X-ray absorption spectroscopy (XAS) at Mn, Fe [Formula: see text]- and [Formula: see text]-edges and O [Formula: see text]-edge. The energy of [Formula: see text]- and [Formula: see text]-edge peaks for Mn and Fe ions stayed unchanged against the change in Mn content [Formula: see text]. The analysis of Mn [Formula: see text]-edge extended X-ray absorption fine structure (EXAFS) revealed that Mn−O polyhedra was kept as an octahedron with Mn[Formula: see text] irrespective of the [Formula: see text] value, agreeing with the results of [Formula: see text]-edge spectra. The Fe [Formula: see text]-edge EXAFS analysis showed that the Fe–O distance decreased with decrease of the Mn contents, suggesting that the ratio of FeO5 polyhedra to FeO6 octahedra is increased due to smaller oxygen amount. By analyzing both the [Formula: see text]-edge and [Formula: see text]-edge spectra, we clarified the local electronic and atomic structural changes particularly occurred in the B-site mixed perovskite oxides.

EXAFS Determination of Clay Minerals in Martian Meteorite Allan Hills 84001 and Its Implication for the Noachian Aqueous Environment

The aqueous environment of ancient Mars is of significant interest because of evidence suggesting the presence of a large body of liquid water on the surface at ~4 Ga, which differs significantly from the modern dry and oxic Martian environment. In this study, we examined the Fe-bearing minerals in the 4 Ga Martian meteorite, Alan Hills (ALH) 84001, to reveal the ancient aqueous environment present during the formation of this meteorite. Extended X-ray absorption fine structure (EXAFS) analysis was conducted to determine the Fe species in ALH carbonate and silica glass with a high spatial resolution (~1–2 μm). The μ-EXAFS analysis of ALH carbonate showed that the Fe species in the carbonate were dominated by a magnesite-siderite solid solution. Our analysis suggests the presence of smectite group clay in the carbonate, which is consistent with the results of previous thermochemical modeling. We also found serpentine in the silica glass, indicating the decrease of water after the formation of carbonate, at least locally. The possible allochthonous origin of the hematite in the carbonate suggests a patchy redox environment on the ancient Martian surface.

The fate of CdS quantum dots in plants as revealed by extended X-ray absorption fine structure (EXAFS) analysis

The use of quantum dots (QDs) is widespread and as such, the potential risk associated with their dispersion in the environment has stimulated research on their interaction with potential sensitive receptors.

EXAFS wavelet transform analysis of Cu-MOR zeolites for the direct methane to methanol conversion

An innovative approach in EXAFS analysis and fitting using wavelet transforms reveals local structure and nuclearity of Cu-species in zeolites.

Solution XANES and EXAFS analysis of active species of titanium, vanadium complex catalysts in ethylene polymerisation/dimerisation and syndiospecific styrene polymerisation

Mechanistic studies through the solution XANES and EXAFS analysis for V and Ti complex catalysed ethylene polymerisation/dimerization, and syndiospecific styrene polymerisation, including interpretation of the XANES spectra, have been introduced.

The abiotic reductive removal and subsequent incorporation of Tc(iv) into iron oxides: a frontier review

This article reviews the mechanism of Tc incorporation into the crystal structure of magnetite and provides comparative EXAFS analysis.